How to animate slices through geometry with a different interior and skin? (e.g. like slicing a watermelon)
Depending on what you want your final outcome to look like you may not need to do anything more then a simple boolean modifier.

Here is a capsule with nothing more then a boolean modifier set to difference, and a animated cube as the target.


You can see it is possible to get a different material for where the boolean cuts the object. Here is how.


Add a second material slot to the object with the boolean modifier (this is the material you want for the "inside" of your object).

The target object (the object that does the cutting) also needs to have the same material.


That's all it takes to get a different material where the boolean modifier has cut.

You can read in further detail about how the Boolean modifier handles materials in the blender manual.